High-pressure food processing, using pressures between 1 and 10 kbar, permits preservation of aroma and vitamins, improvement of textural and sensory properties, and removal of harmful bacteria. The process must be carefully monitored, however, as localized variations in pressure or temperature can significantly influence the removal of bacteria. The process must therefore be uniform, especially in the health-sensitive field of microbial inactivation.

At TU München’s Chair of Fluid Mechanics and Process Control, we are investigating the effect of the fluid dynamics on bacteria during high-pressure food processing. Our research focuses on a 6.3-litre high-pressure chamber that is used to pressurize liquid food contained in five packages, placed on different shelves in the chamber. The chamber is filled with a pressure transferring liquid (in our case water) until the desired pressure level (4 kbar) is reached. The pressure is maintained for about 20 minutes - the time needed to kill the bacteria.

Since experimental methods for investigation at very high pressures, especially for industrial chambers, are not readily available, we have chosen CFD as a means to investigate scale-up and non-uniformity.

The CFD model involves transient compressible flow with conjugate heat transfer. Initially, the fluid is isothermal and at rest. Inflow increases the pressure and temperature (due to conversion of external work into internal energy), until the required compression level is reached. The pressure increase leads to a volume reduction of the packages, which increases the pressure inside. We simulate this by prescribing a transient grid contraction that corresponds to the real package deformation kinematics, although we know that this is a generic fluid-structure interaction problem. The package material is considered as a conducting solid of finite thickness.

Standard CFD-software requires a strong customization to this specific problem; we have to account for an equation of state valid at high pressures, pressure-dependent fluid properties, moving package boundaries, and a scalar transport equation enhanced by a pressure-temperature sensitive model of microbial decay. We have chosen CFX for this application as its highly developed user interfaces, robust solution algorithms and transparent solution process allow the integration of specific algorithms and thus represent an excellent tool for research in this field.

The CFD results have shown that the concentration of the surviving microorganisms depends strongly on the position of the food package in the chamber. In fact, the rate of bacteria decay increases with both temperature and pressure, but as the pressure is spatially constant, the remaining level of bacteria is determined by the spatial temperature distribution. We found higher levels of bacteria in the packages on the lowest shelves due to buoyancy-induced lower temperatures in that area. A low conductive package material improves the uniformity of the processing by preserving the elevated temperature level within the package throughout the pressurization. This yields a better process-uniformity and a lower concentration of surviving bacteria.

With CFX we are currently addressing many different high-pressure applications, including convection effects in high pressure-induced solidification (freezing), dynamics of bacteria populations and fluid structure interaction effects.